Genes have been in the scientific vocabulary for a hundred years. The term "gene" was proposed by the Danish plant scientist Wilhelm Johannsen in the first decade of the 20th century. For Johannsen, the gene remained an abstract concept, "free of any hypothesis", but others were already pointing to chromosomes as the likely location of genes. The science of genetics was born at that time, and genes were rapidly connected with mutations, with patterns of inheritance, with development, with quantitative traits, with evolution and with biochemical pathways. All this was achieved without knowledge of the physical nature of genes, but this changed in mid-century with the discoveries of molecular biology. DNA was revealed as the genetic material, and the mechanisms were elucidated by which the information was encoded, and propagated, and linked to the phenotype. However, the concept of a "gene" did not become clearer. Quite the reverse, as the units of mutation, of recombination, of inheritance, of expression, of regulation, etc. did not necessarily coincide.